Fill in the blanks:
$(1)$ ........... solution is added to produce the precipitate of $BaSO_4$ in the estimation of sulphur.
$(2)$ .......... and .... precipitate will be produced by the estimation of phosphorus.
$(3)$ How many elements are estimated in organic compounds?
$(4)$ ......... gas is produced in the Dumas method,but ....... gas is produced in the Kjeldahl's method.

(N/A) $(1)$ $BaCl_2$ solution is added to produce the precipitate of $BaSO_4$ in the estimation of sulphur.
$(2)$ $Mg_2P_2O_7$ (magnesium pyrophosphate) or $(NH_4)_3PO_4 \cdot 12MoO_3$ (ammonium phosphomolybdate) precipitate is produced in the estimation of phosphorus.
$(3)$ There are $9$ elements estimated in organic compounds: $C, H, O, N, P, S, Cl, Br, I$.
$(4)$ $N_2$ gas is produced in the Dumas method,while $NH_3$ gas is produced in the Kjeldahl's method.